In recent years, a mounting structure having a sub-substrate, which is different from a motherboard, mounted on a motherboard has been proposed. For example, Patent Literature 1 discloses a mounting structure (a substrate for a dielectric filter) in which a second printed board corresponding to the sub-substrate is mounted on a first printed board corresponding to the motherboard. In the mounting structure disclosed in Patent Literature 1, the first and the second printed boards differ from each other in a dielectric constant, a substrate thickness, or a dielectric dissipation factor.
However, in the mounting structure including the sub-substrate mounted on the motherboard, when a heat generating component is mounted on the sub-substrate, there is a problem that a heat dissipation effect is low. Hereinafter, this problem will be described. FIGS. 1 and 2 are diagrams showing a configuration example of the mounting structure of related art. FIG. 1 is a perspective diagram and FIG. 2 is a cross-sectional view showing the configuration example of the mounting structure of the related art. The mounting structure shown in FIGS. 1 and 2 includes a motherboard 110, a sub-substrate 120, and a heat sink 130. The heat sink 130 is bonded to one side of the motherboard 110 over an entire surface thereof, and the sub-substrate 120 is mounted on the other side of the motherboard 110. On the sub-substrate 120, a heat generating component 140 is mounted on a surface of the sub-substrate 120 that is opposite to a surface facing the motherboard 110. The heat generating component 140 is a component that generates heat, and is, for example, a light emitting diode or the like. The heat sink 130 is made of metal such as aluminum, copper, or the like and is provided to dissipate heat generated by the heat generating component 140.
A signal pattern 111 is formed on a surface of the motherboard 110 facing the sub-substrate 120. Further, a ground pattern 112 is formed on the motherboard 110 at a position where the sub-substrate 120 is mounted. Furthermore, through holes 113 are formed in the ground pattern 112. A signal pattern 121 is formed on a surface of the sub-substrate 120 opposite to a surface facing the motherboard 110 and on side surfaces of the sub-substrate 120. Thus, the signal pattern 121 on the side surfaces of the sub-substrate 120 and the signal pattern 111 on the motherboard 110 are electrically connected to each other.
However, in the mounting structure shown in FIGS. 1 and 2, the motherboard 110 and the sub-substrate 120 are interposed in a heat dissipation path extending from the heat generating component 140 to the heat sink 130. For this reason, as thermal resistance of the heat dissipation path includes the thermal resistance of the motherboard 110 and the sub-substrate 120, it becomes very large, thereby lowering the heat dissipation effect.
As a method for enhancing the heat dissipation effect, there is, for example, a method utilizing a structure of the mounting structure (light emitting device) disclosed in Patent Literature 2. In Patent Literature 2, a packaged light emitting diode is mounted on one surface of a wiring substrate corresponding to the motherboard, and a heat dissipation plate corresponding to the heat sink is bonded to an opposite surface of the wiring substrate. Further, an insertion hole is formed in the wiring substrate, and a projection is formed on a surface of the heat dissipation plate facing the wiring substrate. This projection is inserted into the insertion hole of the wiring substrate and is bonded to a bottom surface of the package corresponding to the sub-substrate with a binder such as solder, an adhesive, or the like interposed therebetween. Therefore, a bottom surface of the package is bonded to the projection of the heat dissipation plate with no wiring substrate interposed therebetween. Thus, the wiring substrate is not interposed in the heat dissipation path extending from a light emitting diode chip inside the package to the heat dissipation plate. Accordingly, the thermal resistance of the heat dissipation path can be lowered by the thermal resistance of the wiring substrate, thereby enhancing the heat dissipation effect.